1. Field of the Invention
The present invention relates to a scan circuit in which a plurality of unit circuits are connected and scan pulses are sequentially output from the unit circuits in response to drive pulses.
2. Related Background Art
FIG. 1 is a schematic circuit diagram of a drive section in a solid state image pickup apparatus using a conventional scan circuit.
In the diagram, respective output terminals of a scan circuit 401 are connected to horizontal lines HDL.sub.1, HDL.sub.2, and HDL.sub.3, and to horizontal lines HDL.sub.3, HDL.sub.4, HDL.sub.5, . . . through transistors Qy.sub.1 to Qy.sub.4.
A signal F.sub.1 to select odd number fields is input to gate electrodes of the transistors Qy.sub.1 and Qy.sub.2. A signal F.sub.2 to select even number fields is input to gate electrodes of the transistors Qy.sub.3 and Qy.sub.4 Namely, the transistors Qy.sub.1 to Qy.sub.4 constitute an interlacing circuit.
When the signal F.sub.1 is input, scan pulses Oy.sub.1, Oy.sub.2, . . . are sequentially output to the horizontal lines HDL.sub.1 and HDL.sub.2, horizontal lines HDL.sub.3, and HDL.sub.4, . . . through the transistors Qy.sub.1 and Qy.sub.2.
On the other hand, when the signal F.sub.2 is input, the scan pulses Qy.sub.1, Qy.sub.2, . . . are sequentially output to the horizontal lines HDL.sub.2 and HDL.sub.3, the HDL.sub.4 and HDL.sub.5, . . . through the transistors Qy.sub.3 and Qy.sub.4.
However, in the foregoing image pickup apparatus using the conventional scan circuit, the signals F and F.sub.2 are transferred to the horizontal lines HDL through the transistors Qy.sub.1 and Qy.sub.2 and the transistors Qy.sub.3 and Qy.sub.4 of the interlacing circuit. Therefore, there are problems such that a voltage drop due to a threshold value voltage Vth of the transistor Qy occurs and the dynamic range is narrowed, so that the picture quality deteriorates.
To solve this problem, in Japanese Patent Gazette No. 61-61586, a vertical buffer circuit to compensate a voltage level of the scan pulse Oy is provided. However, since the buffer circuit is separately provided, the number of elements constituting the vertical scan circuit increases and also the buffer circuit has a bootstrap capacitance. Thus, there is a problem such that it is difficult to reduce the area of the elements.
On the other hand, since two horizontal lines are simultaneously driven, the conventional drive circuit cannot be used in the image pickup devices in which a signal is readout by every one vertical line. Namely, the conventional constitution has problems such that there is a limitation in driving manner and this constitution can be applied to only the limited driving method.